1. Field of the Invention
The present invention relates generally to devices and methods for extracting broken bolts or other threaded studs from a tapped hole, and more particularly to novel tool and method for removing a broken bolt, threaded stud, or other threaded member from a tapped hole, the bolt or stud having been broken off above, below or flush with the surface defining the tapped hole.
2. Description of Prior Art
Various devices and methods have been proposed in the prior art for removal of the shank of a broken bolt, threaded stud or the like from a mating tapped hole into which the bolt or stud is threaded. Representative of prior art devices is the familiar "Ezy-Out" tool comprising a tapered metallic member having on its tapered surface a plurality of helical flutes defining edges for engaging the wall of a hole drilled axially into the broken bolt shank so that the shank may be backed out by rotation of the tool. The straight shank or chisel type tools have longitudinal edges for engaging the walls of an axial hole drilled into the broken bolt shank. Other related devices and methods for the removal of broken bolt shanks, studs and the like may be represented by those described in or reference by U.S. Pat. No. 1,227,391 to Cooper describing device and method for removing a pipe wherein an elongate threaded member having a tapered end and a pair of diametrically opposed longitudinal slots is inserted into the pipe and a pair of tapered wedges are tapped into the slots to engage the inner surface of the pipe so that the pipe may be backed out; U.S. Pat. No. 2,670,639 to Flowers et al describing a tool for removing a broken spark plug from an engine cylinder, the tool having an offset for insertion into a hole with an offset drilled into the broken spark plug to back the plug out of the cylinder head of the engine; U.S. Pat. No. 2,684,526 describing a tool having three centering points on one end which when struck against the end of a broken bolt shank provides three indents for drilling holes into the shank for receiving the three pointed tool; U.S. Pat. No. 2,686,447 to Vock et al describing a stud extractor in the form of an elongate tool with an offset defining a key insertable in a corresponding hole provided in the broken stud; U.S. Pat. No. 2,744,311 to Nipken et al and U.S. Pat. No. 2,744,312 to Conger each describing a tool for extracting a broken drill or the like including finger members which are forced down into the flutes of the broken drill inside the hole so that the broken drill may be grasped and removed.
Each of the prior art devices just described suffer from one or more shortcomings seriously impairing utility thereof for the intended purpose. In the prior art devices and methods, the end of the broken bolt shank to be removed must be accessible for drilling a hole for insertion of the extractor tool. In most of the cases, the extractor tool is driven into the drilled hole with sufficient force to engage the wall thereof. Many of the prior art tools have a very short axial extent of engagement with the wall of the drilled hole, and axial alignment of the tool with the drilled hole may therefore be difficult to control with desired precision. The operation of driving the tool into the drilled hole frequently results in deformation of the threads of the tapped hole, in radially outward deformation of the drilled bolt shank against the threads of the tapped hole, and in consequent increased difficulty in backing the broken bolt shank out.